1. Field of the Invention
The present invention relates to a photodiode array, a method of manufacturing the same, and a detecting device. More specifically, the present invention relates to a photodiode array having responsivity in the near infrared, a method of manufacturing the same, and a detecting device, such as an image pickup device or a sensor, including the photodiode array.
2. Description of the Related Art
A photodiode array including an absorption layer composed of InGaAs having responsivity in the near infrared is used in imaging using space light, a sensor for a biologically relevant substance etc. Accordingly, many research and development have been carried out. In such a photodiode array including an absorption layer composed of InGaAs, an electrode (p-side electrode) of each photodiode (pixel) has the following structure. The p-side electrode of each photodiode is connected to a p-type region where Zn (p-type impurity) that has been selectively diffused from the top surface of an epitaxial multilayer is distributed, and further connected to a read wiring of a complementary metal-oxide semiconductor (CMOS) (M. J. Cohen, M. J. Lange, M. H. Ettenberg, P. Dixon, and G. H. Olsen, “A Thin-Film Indium Gallium Arsenide Focal Plane Array for Visible and Near Infrared Hyperspectral Imaging” 1999, IEEE, pp. 744-745). An electrode (n-side electrode) that provides each photodiode with a common ground potential is connected to an InP substrate or buffer layer containing an n-type impurity in a high concentration. The n-side electrode is connected to a ground terminal of the CMOS. Consequently, photogenerated charges generated by being detected by the photodiodes are read by the CMOS and form an image. In this structure, by separately forming p-type regions for respective photodiodes, a p-n junction or a p-i-n junction is independently formed in each photodiode. Accordingly, each of the photodiodes of the above photodiode array does not have a mesa-type structure, in which crystallinity tends to be decreased.
In the case where a p-type region is formed by selective diffusion of Zn, a surface of an n+InP substrate or an n+ buffer layer is exposed, and an n-side electrode is connected to the exposed surface. For this purpose, an edge of an epitaxial multilayer including an InGaAs absorption layer is etched (mesa-etched). The common n-side electrode is connected to a margin (i.e., non-growth part) which is a surface of the exposed n+InP substrate or buffer layer. This n-side electrode is formed so as to extend along a wall of the edge of the mesa-etched epitaxial multilayer and to have the same height as the height of the p-side electrode at the top surface side of the epitaxial multilayer. The n-side electrode is connected to a predetermined terminal of a CMOS, the terminal being arranged in a planer shape. That is, in this photodiode array, each of the photodiodes does not have a mesa-type structure, but the photodiode array has a mesa-type structure.
In the above-described photodiode array including an absorption layer composed of InGaAs, the photodiodes are separately formed by introducing a p-type impurity from the top surface of an epitaxial multilayer, and thus each of the photodiodes does not have a mesa-type structure. However, when the photodiode array is viewed as one unit, the area of the photodiode array forms a mesa-type structure. When an edge of an epitaxial multilayer is mesa-etched, large irregularities due to a difference in the etching rate or the etching plane direction between materials are formed on a wall of the mesa-etched edge. A large number of man-hours are required in this first etching because etchants are changed in accordance with the types of material of each layer constituting the epitaxial multilayer. In order to remove such large irregularities, it is possible to add a correction to the wall of the edge of the epitaxial multilayer by performing additional etching. The additional etching also requires a large number of man-hours. In the case of mass production, performing the above-mentioned first etching and additional etching requires a very large number of man-hours, thereby increasing the manufacturing cost. On the other hand, when only the first etching is performed, and if an n-side electrode is formed on the wall of the edge of the epitaxial multilayer, the wall having large irregularities thereon, the irregularities inhibit, for example, a flow of a gas phase and a smooth deposition of a deposition material of the n-side electrode. As a result, a problem that the n-side electrode is not continuously formed (i.e., disconnection) occurs. It is sufficient that one n-side electrode is formed at any position of the photodiode array, but such irregularities are disposed around the periphery of the edge of the epitaxial multilayer. Accordingly, the problem of disconnection may occur regardless of the position at which the n-side electrode is formed.